1. Field of the Invention
The present invention generally relates to the field of radiation sensors. More particularly, the present invention relates to high sensitivity electromagnetic radiation sensors useful as passive imagers in the millimeter wave or microwave regimes.
2. Description of the Related Art
Radiation sensors are generally known which convert electromagnetic radiation energy absorbed by a radiation sensitive material into an electrical signal. Conventional thermal detection has included temperature sensitive capacitors or temperature sensitive resistors (also called bolometers) arranged in an array where a detector signal is coupled into a signal multiplexer. The amount of radiation received by a thermal detector is relatively small, which is critical, particularly when the detector is an uncooled type, and measures have had to be taken to decrease the heat capacity of the detector and prevent dissipation of the received radiation. To accomplish this, the prior art has used semiconductor fine patterning or micro-machining techniques to fabricate a radiation sensor having a thermally-isolated, thermally sensitive resistive or capacitive material portion suspended as a microbridge structure over a substrate which keeps the thermally sensitive material out of direct contact with the substrate. (See, for example, U.S. Pat. Nos. 4,574,263 and 5,302,933.)
Thermally isolated thermal detectors have been fabricated as an array of microbridges with a thermoresistive (thermo-capacitive) element in each microbridge. In the following, the reference to the "thermoresistive" phenomena or devices will also be understood to include thermo-capacitive phenomena or devices. The resistive microbolometers optimally have a high thermal coefficient of resistance and low thermal conductance between the absorbing area and a readout circuit that multiplexes the radiation signal. As each detector pixel absorbs the radiation being detected or monitored, the microbridge temperature changes accordingly and the elemental resistance is altered. For these arrangements, standard photolithographic techniques with selective etching have been used to pattern the thin film to form detectors for individual pixels of the array of detectors.
However, a problem associated with conventional microbolometer architectures involving thermally isolated detectors is the difficulty in achieving and maintaining an efficient coupling of some bands of electromagnetic radiation, such as millimeterwave (mm-wave) and microwave radiation, to a thermal detector and, in particular, to an array of thermal detectors, while maintaining the thermal isolation of the detector(s) needed for high sensitivity. U.S. Pat. No. 5,450,053, the teachings of which are incorporated herein by reference, describes incorporating antennas in microbolometer detector architectures used for IR/mm-wave detection to provide a mm-wave energy coupling apparatus. In one embodiment, U.S. Pat. No. 5,450,053 describes use of "bow-tie" microantenna designs formed on the backside of silicon substrate while IR sensitive microdetector arrays are formed on the opposite frontside of the substrate, such that incident radiation is collected by the antennas after passing through the substrate.
While U.S. Pat. No. 5,450,053 addresses the issue, a need remains in the art for a radiation sensor with increased sensitivity relative to prior designs.